Obscurant device

ABSTRACT

Many of today&#39;s weapons systems use surveillance and target acquisition (STA) devices which can exploit the infrared and millimeter wavebands of the electromagnetic spectrum. Designing obscurant devices which can provide screening against such systems often results in complicated or costly solutions. A device capable of mitigating these problems is described wherein an obscurant device ( 10 ), and more particularly a device capable of providing screening against the visual, infrared and millimeter wave regions of the electromagnetic spectrum, comprises an obscurant payload, a burster charge capable, when detonated by a detonator, of disseminating said payload and a payload casing wherein some or all of the payload casing is configured to disintegrate upon actuation of the burster charge and to act thereafter as an obscurant.

This application claims priority to Great Britain Application No.9922493.3 filed on Sep. 23, 1999 and International Application No.PCT/GB00/03209 filed on Aug. 21, 2000 and published in English asInternational Publication Number WO 01/22027 A1 on Mar. 29, 2001.

The present invention relates to obscurant devices and more particularlyto those capable of providing screening against the visual, infrared andmillimeter wave regions of the electromagnetic spectrum.

It has long been a desire to increase the survivability of friendlyforces in battle by screening them from enemy sensors. Historically,smoke has been used to achieve this aim. However, advances in the fieldof sensor technology has increased the effectiveness of many weaponssystems by equipping them with surveillance and target acquisition (STA)devices which can exploit the infrared and millimeter wavebands of theelectromagnetic spectrum. Longer wavelength radiation is readilytransmitted through conventional visual obscurant screens therebyexposing friendly forces to greater risks.

Research has shown that there is currently no single material that iscapable of screening effectively at visual, infrared and millimeterwavelengths. Since obscurant materials screen radiation whose wavelengthis roughly equal to their particle size it is highly improbable that asingle material capable of screening across the millimeter to infraredrange will be developed in the near future. In order, therefore, toprovide protection against STA devices it is necessary to deploy amixture of obscurants, for example powders, fibres and pyrotechniccompositions, from a single munition.

There is currently no “commercial off the shelf” device which comprisesa mixture of components designed to counter STA devices. However, adesign for such a device was disclosed in the Smoke/obscurantsSymposium, Apr. 28-30, 1998, Aberdeen Proving Ground, Maryland, USA; TheEvolution of a Design for a Rapid Bloom Multi -Spectral obscurantMunition by P J D Collins, J M B Christofi, N Davies and D Green. Adisadvantage of this design is that it would tend to be relatively largeand complex and therefore expensive to manufacture. A furtherdisadvantage of this device is that the munition has a section with acalibre that is larger than the standard US and UK calibre.

The only known millimeter wave screening munition is the United StatesM81 66 millimeter grenade (NATO Classification; Grenade Launcher Smoke:MM/IR screening M81). A disadvantage of this grenade is that, althoughthe design is capable of carrying some infrared screening payload, it isoptimised for performance in the millimeter waveband. In practice, inorder to achieve multi-spectral screening the US require the use of anumber of different obscurant devices, e.g. one for infrared screening,one for visual screening and one (the M81) for millimeter screening.

It is therefore an object of the present invention to provide anobscurant device which alleviates some of the above disadvantages byconstructing part or all of the device payload casing from a materialthat contributes to the screening effect of the device.

Accordingly, the present invention provides an obscurant devicecomprising an obscurant payload, a detonator, a burster charge which isinitiated by action of the detonator and which is capable ofdisseminating said payload and a payload casing wherein some or all ofthe payload casing is configured to disintegrate upon actuation of theburster charge and to act thereafter as an obscurant.

Usefully the payload casing can be configured to provide effectiveelectromagnetic screening in the millimeter waveband by constructing thecasing out of a conductive carbon fibre. In this context effectivemillimeter wave attenuation is taken to be ≧10 dB (≦10% transmission)for a single pass through an obscurant cloud.

Suitable fibre types for construction of the casing include:

i) UTS carbon fibre, a PAN (poly-acrylo-nitrile) based carbon fibrewhich has a Young's Modulus (YM) of 230 Gpa;

ii) Nickel coated carbon (Ni—C), a PAN based carbon fibre with a YMsimilar to UTS;

iii) UD cloth carbon (UD-C), a unidirectional non-crimp material usingcarbon with a YM=230 Gpa;

iv) J-UTS carbon fibre, similar to the UTS fibre above but with a higherstrain to failure;

v) P100s carbon fibre, a pitch based carbon fibre with higher electricalconductivity than that observed for PAN-based fibres;

vi) Ultra-high Modulus (UMS) carbon fibre, a high modulus PAN-basedcarbon fibre.

It was found in tests that highest mean attenuation in the measuredmillimeter wavelengths was achieved when the casing was made from VMScarbon fibre.

In order to achieve attenuation at the required frequencies theconductive casing should disintegrate into fibre lengths in the range of1 mm to 10 mm. This is because the level of attenuation is maximisedwhen the fibre-length is approximately a half-wavelength. For example,at 94 GHz (=3 mm) a fibre length of 1.5 mm is required.

Furthermore, manufacture of the payload casing can conveniently beachieved by dry filament winding as described more fully hereinafter.The Applicant has found that manufacture of the payload casing by theabove technique using commercially available carbon fibre naturallyresults in a structure that disintegrates upon detonation intoindividual fibres suitable for millimeter screening. Suitable carbonfibre can be obtained from, for example, the following companies TenaxPlastics Limited, Akzo, Amoco, Courtaulds and Roskill.

Conveniently, the device can carry a mixture of obscurants as payload inorder to result in screening at multiple wavebands. For example, if thedevice carries a brass flake/red phosphorous payload then, in additionto the millimeter screening effect generated by the disintegratingpayload casing, the device also screens in the infrared and visualwavebands.

A device as described above can conveniently be adapted for use as amunition or as a decoy flare for deployment from an aircraft or a ship.At present aircraft and ships use different infra-red and radar decoys.For use in aircraft the device described above would be loaded with amagnesium/teflon/viton (MTV) payload and for naval uses a payload of redphosphorous would be appropriate.

An embodiment of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, wherein

FIG. 1 shows a cross-section of a device design according to theinvention

FIG. 2 shows attenuation against time plot for the attenuation of the KBand (35 GHz) radiation for trial 1 (wind speed conditions <2 msh⁻¹).

FIG. 3 shows attenuation against time plot for the attenuation of the MBand (94 GHZ) radiation for trial 1.

FIG. 4 shows the same attenuation versus time plot as FIG. 2 but fortrial 2 (wind speed conditions between 7 ms⁻¹ and 9 ms⁻¹).

FIG. 5 shows the same waveband versus time plot as FIG. 3 but for trial2.

FIGS. 6 and 7 show the transmission against time at 5 specificwavelengths in the visual and infra red wavebands for trials 1 and 2respectively.

Referring now to FIG. 1. This figure shows a cross section throughtypical multi spectral obscurant device 10. In this embodiment thepayload, brass flake 20, is contained within a spool 30 sealed with endcaps 31, 32. The end caps 31, 32 have apertures through which tube 33 isfitted along the axis of the spool 30. Tube 33 is sealed to the end caps31, 32 and to the spool 30 and contains high explosive pellets 40comprising <95% RDX (Hexahydro-1,3,5-trinitro-1,3,5-triazine), such asDebrix High Explosive pellets as manufactured by Royal Ordnance. Adetonator 60 is located at one end of tube 33 and is connected to a fuzeor firing box (not shown) by leads 61.

The conductive UMS carbon fibre case 50 surrounds the spool and is addedby dry filament winding (The process of dry filament winding involveswinding the fibre off a reel, at a set fibre tension. The fibre is thenpassed through a winding eye and is finally wound onto a bobbin, i.e. inthis case the spool). During construction of the payload the fibre isinitially wound onto itself a number of times in order to anchor itselfto the spool. A pre-programmed is winding program is then run until thedesired mass is deposited onto the spool. The free end of the fibre isthen bonded to the deposited fibre by using an adhesive.

Alternatively the carbon fibre case 50 could be constructed separately.The components of the obscurant device 10 could then be assembled andthe payload loaded into the device.

In operation, an electric pulse from a fuze or firing box initiates thedetonator 60. The exploding detonator 60 produces a shock wave whichdetonates the high explosive pellets 40. The detonation of the highexplosive pellets 40 disseminates the payload, brass flake 20, and alsocauses the carbon fibre case 50 to disintegrate and to act thereafter asa millimeter waveband obscurant.

During trials carried out on the obscurant device over 55 devices weretested. In all cases the conductive fibre casing had a diameter of 66millimeters and was 160 millimeters in length. The particular carbonfibre used had a diameter of 7 microns. On average the total weight ofthe device with the carbon fibre casing was 1157 grams (this valuevaried from around 1100 to 1200 grams across the tested devices). Theaverage weight of carbon fibre casing was 159 grams (this value variedbetween 99 and 183 grams). Twelve Debrix pellets were used as theburster charge.

FIGS. 2 to 7 represent results which are typical of all the testeddevices and as can be seen from FIGS. 2 to 5 the carbon fibre casinggenerates an efficient obscurant field in the millimeter wavebands.

Turning to FIG. 2 it can be seen that significant attenuation of the Kband is achieved almost immediately following device detonation. Over 20dB attenuation is recorded for the first ten seconds. This drops toaround 8 dB for a few seconds before returning to 20 dB for another fiveseconds. FIG. 3 shows that a screen of over 40 dB was initially formedin the M band and that this screen reduced to around 15 dB after eightseconds. It is therefore clear that significant attenuation within themillimeter waveband is achieved under low wind speed conditions by usingthe invention.

Turning to FIGS. 4 and 5 it can be seen that even under higher windspeed conditions an obscurant cloud capable of attenuating along theline of sight is generated.

Effectiveness of the generated visual/infrared obscurant cloud is notcompromised by using the casing to generate the millimeter obscurantfield. This can be ascertained by examination of the visual and infraredtransmission data as detailed in FIGS. 6 and 7. It can be seen thattransmission at each of the five wavelengths monitored is immediatelyreduced to low levels once the device detonates. Effective obscurationvaries from 8 to 30 seconds depending on the wind conditions (i.e. highwind to low wind speed).

Further embodiments of the invention can be envisaged wherein differentobscurant materials are used as the payload, i.e. red phosphorous ormagnesium/teflon/viton (MTV).

What is claimed is:
 1. An obscurant device comprising an obscurantpayload, a detonator, a burster charge which is initiated by action ofthe detonator and which is capable of disseminating said payload and apayload casing wherein some or all of the payload casing is configuredto disintegrate upon actuation of the burster charge and to actthereafter as an obscurant.
 2. The obscurant device as claimed in claim1 wherein some or all of the payload casing disintegrates upondetonation to form a millimeter waveband obscurant.
 3. The obscurantdevice as claimed in claim 2 wherein the payload casing is made of aconductive carbon fibre.
 4. The obscurant device as claimed in claim 3wherein the conductive fibre is selected from the group consisting ofUTS, Ni—C, UD-C, J-UTS, P100s and Ultra-high modulus carbon fibre. 5.The obscurant device as claimed in claim 4 wherein the payload casing ismade of Ultra-high modulus carbon fibre.
 6. The obscurant device asclaimed in claim 2, wherein the payload casing disintegrates upondetonation into fibre lengths of between about 1 mm and about 10 mm. 7.The obscurant device as claimed in claim 3, wherein the payload casingis constructed by carbon fibre winding.
 8. The obscurant device asclaimed in claim 3, wherein the carbon fibre has a diameter of about 7microns.
 9. The obscurant device as claimed in claim 1 wherein theobscurant payload is capable of providing obscuration at visualwavelengths.
 10. The obscurant device as claimed in claim 9 wherein theobscurant payload comprises red phosphorous.
 11. The obscurant device asclaimed in claim 1 wherein the obscurant payload is capable of providingobscuration at infrared wavelengths.
 12. The obscurant device as claimedin claim 11 wherein the obscurant payload includes brass flakes.
 13. Ascreening decoy flare suitable for deployment from an aircraftcomprising an obscurant device as claimed in claim 1 carrying a payloadof magnesium/Teflon/viton.
 14. An obscurant device as hereinbeforedescribed with reference to the accompanying drawings.
 15. An obscurantdevice comprising an obscurant payload, a detonator, a burster chargewhich is initiated by action of the detonator and which is capable ofdisseminating said payload, and a payload casing having a substantialportion configured (i) to disintegrate upon actuation of the burstercharge and (ii) thereafter to act as an obscurant providing effectivescreening in at least part of the electromagnetic spectrum.
 16. Anobscurant device according to claim 15 in which the disintegrableportion of the payload casing disintegrates upon actuation of theburster charge into a plurality of pieces having length approximatelyone-half wavelength of a part of the electromagnetic spectrumeffectively screened.